[ecash-herald] patch docker

Summary:
fix capitalization mismatch

in fixing the lint error thrown on WasmBuilder, did not update all instances of it

for some reason ... builds do not fail in this case. tho the app won't work.

Test Plan: docker build -f ecash-herald.Dockerfile -t ecash-herald_local .

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17358